While you play on normal, the looting system seems pretty random and balanced on the bosses difficulty. But once you did all the bosses on normal and do it again on hard, you realise that its the same loots regardless of the difficulty set. I suggest that the difficulty of the boss affect the loots from it.

Someone told me (can't remember who sadly) it would be interesting if the loots from the bosses were based on the bosses types. For exemple, prismatic items (which seems pretty protoss to me) would only drop when killing a protoss boss.Hybrids could be both zerg and protoss, or have their own item drops.